Merge old contributing.md doc into the developers docs section

docs/developers.rst

diff --git a/docs/developers.rst b/docs/developers.rst
index 22df181..41bc029 100644
--- a/docs/developers.rst
+++ b/docs/developers.rst
@@ -6,6 +6,15 @@
 Developers
 ==========
 
+Contributions are very welcome on this project. Issues should be raised for bugs and feature
+requests. Pull requests should be targeted at the master branch and will run against CI.
+
+.. note:: The rules in the repo are generated by the code in tools/rulegen and the output rules
+  should therefore not be edited directly. However, if you are not comfortable editing this code,
+  please submit a PR with just the output files edited and we'll help update the generator to get
+  the desired result.
+
+
 Code Layout
 -----------